The present invention relates to demodulation and channel estimation in a wireless communication system, and more particularly, to systems using mixed slot formats. Even more particularly, the present invention relates to demodulation systems using both coherent eight (8) Phase-Shift-Keying (8-PSK) modulation schemes, as well as Differential-Quadrature-Phase-Shift Keying (DQPSK) modulation schemes, wherein a new slot format is defined which frees up bits priorly used for a reference symbol.
Prior IS-136+ standards supporting modulation formats used both a standard SYNC pattern as defined in IS-136 Rev. A, as well as a standard reference (REF) symbol, or reference symbol directly following the SYNC pattern in the slot format. Due to backward compatibility issues, IS-136+ slot is required to have exactly the same SYNC pattern as IS-136A slot, i.e., SYNC pattern symbols need to be differentially modulated (DQPSK) while other data are coherently modulated (8-PSK). Therefore, it was originally considered necessary to insert the reference symbol (REF) between this SYNC pattern and a data field in order to resolve phase reference inconsistencies since a random phase shift from the SYNC pattern is an unknown quantity to the receiver. The REF pattern was checked against the SYNC pattern for any phase differences and was used to demodulate a coherent 8-PSK-modulated IS-136+ slot. This is the basis for the slot format used in the IS-136+ down link slot format accepted by the Telecommunications Industry Association (TIA) in April 1998.
The IS-136 Rev. A standard is an earlier standard than the April ""98 IS-136+ standard and uses 2 bits per symbol instead of 3 bits per symbol respectively. Another difference between IS-136 Rev. A and the April ""98 IS-136+ standards, is that the former uses Differential-Quadrature-Phase-Shift-Keying (DQPSK), which allows for only 2 bits per symbol as opposed to the eight (8) Phase-Shift-Keying (8-PSK) modulation scheme (IS-136+ standard) using 3 bits per symbol. In the 8-PSK scheme, eight (8) modulation values are possible using a 3 bit symbol instead of only four (4) modulation values available from the 2 bit symbol of the former IS-136 Rev. A standard.
In a typical slot format using IS-136-TDMA North American Standard (NAS), a 40 millisecond frame, has six (6) slots of 6.67 milliseconds each slot, and 162 symbols comprising a SYNC pattern, pilot data (fixed symbols used for channel condition estimation) and data.
An inherent problem in utilizing the SYNC symbols plus REF symbol to do demodulation and channel condition estimation, is that, data bandwidth utilized by the REF symbol cannot be used for transmitting data bits or link control information in the data field of the slot format. Another problem is that demodulation with the current scheme depends on correctly decoding the REF symbol so as not to corrupt data in other data fields. An entire burst can be lost due to a single symbol error when depending upon the REF symbol to correctly demodulate the rest of the slot. Therefore, eliminating the need for the REF symbol and improves overall receiver performance as well as system performance. Once the REF symbol is removed, the freed-up 3 bits (or symbol) can be reserved for any purpose.
To achieve necessary backward compatibility between IS-136 Rev. A and IS-136+ mobiles the SYNC pattern needs to be differentially encoded in each case. This causes a random phase shift depending on the last symbol phase of the previous slot. Therefore, since the SYNC pattern (symbols) does not use a fixed phase reference for transmission and the rest of the data fields use a fixed referenced phase, a reference inconsistency exists due to the random phase shift of the SYNC being unknown at the receiver. Backward compatibility therefore requires finding a way to determine or compensate for this unknown random phase shift of the SYNC at the receiver.
The present invention advantageously addresses the above and other needs.
The present invention advantageously addresses the needs above as well as other needs by providing an apparatus and method of performing demodulation for transmitted data in a slot format which compensates for an unknown random phase shift of a SYNC pattern at a receiver other than by relying upon a standard reference (REF) symbol. Compensation occurs by essentially pre-rotating phases of all symbols to be transmitted at the transmitter such that all the symbols have equal reference phases. This is possible since a receiver cannot distinguish sources of phase shifts on a received signal.
A method of demodulation in a system for a mixed slot formation including differential and coherently encoded slots includes: pre-phase encoding data and a predetermined sequence in a slot at a transmitter such that the encoded data and the predetermined sequence have equal reference phases from a reference symbol of a previous slot.
In a variation, the method further includes: pre-phase encoding the predetermined sequence with a phase determined from a reference symbol from the previous slot at the transmitter, if the previous is in IS-136A format.
For a variation wherein the previous slot is in IS-136+ format, the method further includes differentially phase encoding the predetermined sequence according to IS-136A standards using a reference phase of       π    4    .
In another variation, pre-phase encoding data includes phase shifting data using one of a group of different phase angles, each of the different phase angles an integer multiple of       π    2    .
In another embodiment, an IS-136 compatible receiver utilizing a 8-Phase Shifting-Key (8-PSK) slot structure for downlink communications comprises: a receiver for receiving a plurality of slots having a slot structure comprising: a SYNC sequence, data bits, and a plurality of reserved bits.
In a further variation of the receiver the plurality of reserved bits is a data field.
In yet another variation of the receiver the plurality of reserved bits is a link control field.
In another embodiment, an IS-136 compatible transmitter comprises: a differential encoder to differentially encode a SYNC pattern in a slot if a previous slot is in IS-136 Rev. A format; and a phase-mapping encoder to phase-shift data bits by an amount determined by mapping a previous reference symbol to a multiple of       π    2    .
A further embodiment comprises a transmitter having means for generating a plurality of slots with a slot format comprising: a SYNC sequence, data bits and reserved bits.